The fact that he hasn't commented about his poster would suggest that. If he knew Estarossa shouldn't he be alarmed, after all it could suggest that someone saw Estarossa who is supposed to be sealed? If he didn't recognize him at all this would be the sign that he didn't know about it beforehand.
And then there is the brother thing. Who is older? If it's Estarossa then maybe he shouldn't feel so guilty towards Zeldris, that is if those three are really brothers. It could be that Zeldris calls Estarossa brother but it's not literal, and yet we have those two Mel and Estarossa who look alike. This situation is like the Dreyfus/Fraudrin one, so rather than having two people maybe it's accurate to think that Mel and Estarossa are or were one person at some point.
Those posters get reissued every year, any shock he had from seeing it would have happened long before the manga even started, nor would he have had any reason to bring it up since, as Mel has always hidden his true nature from everyone else.
Of course it is the symbol of the goddess clan. Elizabeth, the apostle of the goddess, got one of those on her eye once she activated her powers. It seems to be inherent to the goddess folk as far as I can tell...
That just means they are using the symbol as it is commonly used.
The triskelion is commonly used to represent constant forward movement/growth of power.
It has also been attributed to trinity such as
-Creator, Destroyer, Sustainer
-Creation, Preservation, Destruction
-The Otherworld: Where spirits, gods and goddesses live.
-The Mortal World: Where you and I live along with plants and animals.
-The Celestial World: Where unseen energies live and move about. Like the forces of sun, moon, wind and water.
Which makes it thematically appropriate for both the demons and the goddesses.